1. Technical Field
The present disclosure relates to an antenna device including a radome, a wireless communication apparatus including such an antenna device, and a radar apparatus including such an antenna device.
2. Description of the Related Art
In recent years, rising attention has been paid to preventive safety technology to prevent accidents from occurring. For example, ACC (Adaptive Cruise Control), which uses 76-GHz millimeter-wave radar apparatuses, and ADAS (Advanced Driver Assistance System), which includes pre-crash safety systems, have been being included as standard equipment. In response to increasing demand for the preventive safety technology, the standardization of a new frequency band (79-GHz band) for radar apparatuses is being promoted. The new frequency band is expected to be available from fiscal 2015.
A millimeter-wave radar apparatus transmits millimeter radio waves (radar waves) in a desired direction, receives reflected waves from an object, and thereby detects in advance an object that may pose an impediment. In an on-board millimeter-wave radar apparatus, the transmission and reception of radar waves are performed, for example, by using a flat patch antenna or a slot antenna formed in a waveguide. A conventional antenna is installed on an exterior body of an automobile through which radar waves pass, in particular on an inner side of a bumper that faces in the direction that the automobile travels. Further, the inner side of the bumper of the automobile is not an enclosed space and therefore admits rain, dust, and the like. Therefore, a millimeter-wave radar apparatus is provided, for example, with a radome in a fixed part of the apparatus to which an antenna is attached. The radome stands in front of the antenna to protect the antenna. The radome has certain degrees of thickness and strength to ensure durability and sealing performance.
In a conventional millimeter-wave radar apparatus, radar waves radiated from an antenna element pass through a radome and are radiated toward an object to be searched for, and reflected waves from the object to be searched for pass through the radome again and arrive at the antenna element. During passage through the radome of the radar waves and the reflected waves from the object, some of the radar waves are reflected by an inner surface of the radome (a surface of the radome that is close to the antenna element) and an outer surface of the radome (a surface of the radome that is remote from the antenna element) due to the difference in wave impedance between the radome and an air layer. The reflected waves reflected by the radome affect a beam pattern of the radar waves. As a result, the conventional millimeter-wave radar apparatus induces a decrease in antenna gain and an increase in side lobe level. For this reason, Japanese Patents Nos. 4065268 and 3419675 propose methods for determining the thickness of a radome on the basis of the electrical length of the propagation path of radar waves in the radome.
The methods proposed in Japanese Patents Nos. 4065268 and 3419675 are effective in a case where the direction of main lobe radiation of radar waves is fixed. However, the methods proposed in Japanese Patents Nos. 4065268 and 3419675 are not effective in a case where the direction of main lobe radiation of radar waves is changed. This is because radar waves are such that the electrical length of the propagation path of the radar waves in the radome varies depending on the angle at which the radar waves enter the radome. Therefore, in a case where the direction of main lobe radiation of radar waves is changed, a radome whose thickness has been determined by either of the methods proposed in Japanese Patents Nos. 4065268 and 3419675 has difficulty in restraining a reflection loss of radio waves from being caused by the radome.
Therefore, an antenna device is required to prevent a decrease in antenna gain and an increase in side lobe level from being caused by a radome.